Method to improve the formation of a paper or paperboard web by pre-pressing in a twin-wire former

ABSTRACT

A method and apparatus for improving forming of a web of paper or paper board employing prepressing during web forming. Forming wires are brought together over at least one roll or alternatively two rolls. The web then passes, sandwiched between two wires through one or more pressure nips which may be one or more roll nips or an extended nips. The pressure nip may be formed as the web travels between the two fabrics, one of them being a forming wire, along a straight path or alternatively as the web travels partially around a roll. The web is then sandwiched between a wire and a fabric, which may be a felt or a belt, and passed through another press nip which may be one or more roll nips or an extended nip.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and apparatus for forming a web of paper or paperboard from stock using a twin-wire former.

BACKGROUND OF THE INVENTION

[0002] A paper making machine typically includes a forming section where a head box ejects a flow of a paper or paperboard stock suspension between adjacent surfaces of a pair of endless fabrics or metal screens, referred to as wires. As the stock suspension travels along with and between the wires, water is removed by gravitational and centrifugal forces. Water removal can be further increased by applying vacuum or pressure pulses. The forming section is followed by a press section where the web and at least one water receiving felt is lead through at least one press roll nip. The web is then transported to a drying section of the paper machine where the web is dried and from there to other sections of the paper machine where the web is further treated, such as by being coated. Such a forming section is typically referred to as a twin-wire forming section. Examples of twin-wire forming sections are disclosed in U.S. Pat. Nos. 3,726,758, 4,056,433, and 4,879,001.

[0003] In papermaking, it is a goal to form a web in a forming section of a paper machine that has a uniform consistency throughout its length and width, that is, at all points of the web the density of the pulp fibers is consistent. One known way of forming such a uniform web is to increase the water content of the fiber suspension that is ejected from the head box. Such a fiber suspension is referred to as having a low consistency, typically having a fiber content of about 1% or less. By using a low consistency fiber suspension, the fibers in the suspension are more free to flow as the wires gradually press them to form the web. However, using a low consistency fiber suspension requires a great deal of water which must subsequently be removed before a web of dry paper or paperboard is obtained. To remove such great amounts of water requires that the forming section include additional dewatering elements which increase its cost, both in terms of the initial investment and in terms of the cost to operate the machine. Furthermore, pumping equipment must be sufficient to handle the high volume of water used in forming the fiber suspension and the high volume of water subsequently removed from the web. The capital and operating cost of such pumping equipment also increases the overall cost of the paper machine.

[0004] If instead a high consistency fiber suspension is employed, that is, having a fiber content of greater than about 1% and as high as about 2% to about 3% or as high as about 12%, less water is used and less water has to be removed during the paper making process, thereby decreasing the capital and operating cost of the process. One fundamental characteristic of a high consistency formed web is that its fibers are more or less randomly oriented in all directions rather than in the plane of the web. A likely reason for this phenomenon is that during drainage the densely packed fibers collide with one another and cannot easily change their orientation from vertical to horizontal. The result is that, rather than forming a planar structure, the fibers form a felted structure with high bulk, a random fiber orientation, high porosity, a grainy formation, increased z-direction strength, and reduced in-plane strength. Thus, fiber orientation affects the strength of the paper and additionally determines whether the paper has the undesirable tendency to curl. Additionally, in high consistency forming, flocks or agglomerations of fibers tend to form which have typically been difficult to break up. If these flocks are not broken up, it is likely that the paper or paperboard ultimately formed will have varying density, leading to a product without a uniform opacity and/or thickness. While many of these characteristics are either desirable or immaterial for some board grades, they are generally not desirable for paper. High consistency forming has been used to make 100 g/m² or heavier board; however, no one has yet developed a suitable high consistency former for making paper.

[0005] To remove water from the web in the forming section, foils have traditionally been used which contact and press the forming wires together. However, because such foils make a frictional contact with the forming wires, they will necessarily cause wear to the wires, thereby decreasing their useful life. In addition, because the frictional forces applied by the foils to the forming wires must be overcome by the equipment that move the wires, energy use is increased. Vacuum is also frequently used to assist in dewatering in the forming section. However, vacuum devices also result in wear to the forming wires and require a great amount of energy for their operation.

[0006] The prior art shows various methods of dewatering the web at the end of a web forming section. In U.S. Pat. No. 3,285,806, a press roll is positioned at the end of the forming section and presses the web between the forming wire and a water receiving pick-up felt. U.S. Pat. No. 3,671,389 discloses pressing the web with a press felt in a forming section after the web has been removed from the forming wire. U.S. Pat. No. 4,056,433 discloses a twin-wire former in which a bottom wire leads the web into the press section where the first press nip is between the bottom wire and a felt. U.S. Pat. No. 4,879,001 discloses a twin-wire former in which the bottom wire leads the web into a press nip in which a felt is on one surface of the web and the bottom wire and a second felt are on the other surface of the web. U.S. Pat. Nos. 5,389,205, 5,522,959 and 5,620,566 and PCT Patent Publication No. WO 98/32917 disclose the use of a roll nip press or an extended nip press at the end of the forming section in which the web supported by a bottom wire passes through the extended nip press along with a felt or wire applied to the upper surface of the web. U.S. Pat. No. 5,820,731 discloses one or two press rolls are positioned at the end of the forming section to press the web between the forming wire and another wire. German Patent Document No. DE 196 54 200 discloses the use of prepressing in a forming section with a dry content of less than or equal to 15 %. PCT Patent Publication No. WO 97/13030 discloses prepressing the web in a press nip employing a water impervious transfer belt. However, none of these references teach that the use of prepressing of the web in the forming section can be used to improve the characteristics of the web, such as formation and/or fiber orientation.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, a web of paper or paper board is formed by ejecting a suspension of paper pulp stock from a head box between a pair of forming wires. The forming wires are brought together over at least one roll or alternatively two rolls. The web thus formed then passes, sandwiched between two wires (which may be the same two forming wires or only one of the forming wires and a different fabric, that may be a wire, a felt or a belt) through one or more pressure nips. The pressure nip may be one or more roll nips or an extended nip in which the web is pressed along a portion of its length in the extended nip press. The pressure nip may be formed as the web travels between the two forming wires along a straight path or alternatively as the web travels partially around a roll. The web is then sandwiched between a wire and a felt or a belt and passed through another press nip which may be one or more roll nips or an extended nip. The linear load in the press nips is preferably between about 50 kN/m to about 1000 kN/m.

[0008] To aid in contouring the web across the width of the web, one or more of the press rolls in the nips may optionally be selected so that the roll's shell is adjustable along its length. Although foils may be used to aid in forming the web, fewer foils are required or may even be omitted entirely. Less vacuum is required to assist in dewatering of the web. Consequently, the power required to drive the wires may be reduced, and the frictional wear upon the wires is substantially eliminated, thereby increasing their useful life.

[0009] Because the press nips are capable of removing the vast preponderance of the water from the web, the length of the forming section may be reduced and the subsequent drying section may be simplified since fewer drying elements are required, thereby resulting in a cost and space savings.

[0010] The present invention also allows the use of a high consistency fiber suspension, of from about 2% to about 3% and as high as about 10% (as compared with a conventional consistency of about 1% or less), while at the same time producing a web of substantially uniform consistency and with minimal formation of flocks or agglomerations of fibers. Consequently, the problems associated with low consistency fiber suspensions are substantially eliminated—high water usage, a requirement for high pumping capacity for the fiber suspension and the water removed from the web, and a requirement for additional drying equipment in the drying section of the paper machine.

[0011] The present invention is not only capable of providing substantial dewatering of the web but in addition providing a web with an improved structure. By applying pressure with one or more press nips in the forming section, formation can be considerably improved to result in more uniform fiber orientation in the plane of the web, increased strength of the paper in the plane of the web, and a decrease in the tendency of the finished paper to curl.

[0012] The present invention can be used in conjunction with the manufacture of all paper and paper board grades, and can be used in new paper machines or as a retrofit to or rebuild of existing machines. Web speeds of about 400 m/min to at least about 2700 m/min are achievable to ultimately form a web with a basis weight of from about 30 to about 200 gmm².

[0013] Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the drawings, wherein like reference numerals delineate similar elements throughout the several views:

[0015]FIG. 1 is a schematic drawing of a web forming section of a paper making machine in accordance with a first embodiment of the present invention;

[0016]FIG. 2 is a schematic drawing of the embodiment of the present invention shown in FIG. 1 and showing additional embodiments thereof;

[0017]FIG. 3 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0018]FIG. 4 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0019]FIG. 5 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0020]FIG. 6 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0021]FIG. 7 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0022]FIG. 8 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0023]FIG. 9 is a schematic drawing of a web forming section of a paper making machine in accordance with other embodiments of the present invention;

[0024]FIG. 10 is a schematic drawing of a web forming section of a paper making machine in accordance with another embodiment of the present invention;

[0025]FIG. 11 is a schematic drawing of a web forming, section of a paper making, machine in accordance with another embodiment of the present invention;

[0026]FIG. 12 is a bar chart showing experimental results employing prepressing, in accordance with the present invention;

[0027]FIG. 13 is a graph showing experimental results of formation employing, prepressing, in accordance with the present invention; and

[0028]FIG. 14 is a graph showing experimental results of fiber orientation employing prepressing in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0029]FIG. 1 schematically depicts a web forming section of a paper making machine in accordance with a first embodiment of the present invention. The forming section includes an endless first forming wire 4 and an endless second forming wire 6 which are disposed adjacent a head box 2 so that paper or paperboard stock suspension ejected from the head box 2 is fed between the first and second forming wires 4, 6. The first and second forming wires 4, 6 travel toward the head box 2, as shown by the arrows, over a plurality of guide rolls 8. The stock suspension is ejected substantially vertically upward from the head box 2 between the converging surfaces of the first and second wires 4, 6 as they travel about rolls 10, 12 which bring the two forming wires 4, 6 together to have a common run. The forming wires 4, 6 with the web sandwiched therebetween travel from roll 10 to roll 16 in a substantially vertical path. Rolls 10, 12 and 16 are preferably perforated or grooved so that water passing through the forming wires 4, 6 can travel through or about the rolls 10, 12, 16. Rolls 10 and 16 preferably have one or more vacuum zones where vacuum is applied to forming wires 4 and 6, respectively. The vacuum zones are preferably individually controlled so that different vacuum levels may be employed in each vacuum zone. The forming wires 4, 6 are pressed together in their substantially vertical common run due to the tension in the wires 4, 6 and the path traveled by the forming wires 4, 6 between roll 10 and roll 16. In particular, as the pair of forming wires 4, 6 pass around roll 10, the wires are pressed together, thereby expelling water from the fiber suspension to form a paper or paperboard web. A plurality of foils 14 are optionally positioned along the common run of the two wires 4, 6 between roll 10 and roll 16 to further press the wires 4, 6 together to remove water from the web. Some or all of the foils 14 may be backed by blades 20 which aid in pressing the two forming wires 4, 6 together. In addition, some or all of the foils 14 may be vacuum foils to further increase the amount of water that is removed from the web. The wires 4, 6 with the web sandwiched therebetween are then partially wrapped about roll 16. By having the pair of wires 4, 6 pass partially around roll 16, the wires 4, 6 are pressed together to expel water from the web by centrifugal dewatering and in addition the fibers are pressed together to form a more cohesive web. A press roll 18 is positioned so that it further presses the two forming wires 4, 6 together, thereby pressing the web in a press nip N₁ against roll 16, thereby removing water from the web and improving the fiber structure of the web.

[0030] After the web, sandwiched between the two forming wires 4, 6, has passed through the press nip N₁ formed by rolls 16 and 18, additional water is removed from the web by suction box 22 and foils 24, which may be vacuum foils to provide additional water removal. The first forming wire 4 is then removed from the web by a guide roll 8, and an endless fabric 26, guided by a guide roll 32, is applied to the web by a transfer roll 30 backed by a support roll 28, which is preferably a grooved or perforated roll. The nip N₁ formed by support roll 28 and transfer roll 30 is preferably a press nip so that the web W will adhere to the bottom side of the fabric 26 and can'thus be removed from the second forming wire 6. In addition, when the nip N₂ formed by support roll 28 and transfer roll 30 is a pressure nip where pressure is applied to the web, additional water is removed from the web in the nip and the smoothness and the structure of the web is improved. The web W is then removed from the second forming wire 6 by adhering to the bottom surface of the fabric 26 for further pressing and drying in the drying section of the paper machine (not shown). To aid in the transfer of the web from the second forming wire 6 to the fabric 26, support roll 28 can be perforated or foraminous and has a plurality of air pressure zones where positive air pressure is applied through the second forming wire 6 to the web W so that the web will more easily be removed from the second forming wire 6. If the fabric 26 is a felt, the transfer roll 30 is preferably a pick-up roll having one or more vacuum zones. Alternatively, the fabric 26 may be a transfer belt having a surface to which the web adheres. Such a transfer belt is described, for example, in U.S. Pat. No. 5,298,124, which is incorporated herein by reference. When a transfer belt is used, the transfer roll 30 preferably has a solid surface.

[0031] One or more of the rolls in one or more of the various nips may alternatively be covered with a shrink-sleeve fabric jacket or sleeve to form a shrink-sleeve press. In accordance with the embodiment, the underlying roll may have a smooth surface (with either a metal, rubber ore rubber-like surface), a grooved surface or a perforated surface. Although in FIG. 1 the foils 14 are shown as being disposed on only one side of the forming wires 4, 6, foils may alternatively be positioned on both sides of the wires 4, 6. Furthermore, more or fewer foils and blades than as shown in FIG. 1 may be used in accordance with the present invention. In fact, to substantially eliminate friction upon the forming wires and thereby increase their useful life, the foils and blade may be completely eliminated. In accordance with this embodiment of the present invention, which is shown in FIG. 2, water removal from the web and forming of the web is accomplished by the tensioning and the pressing of the forming wires along their path of travel in conjunction the pressing of the web between the forming wires as they pass through the pressure nips, N₁ and N₂.

[0032] The embodiment of FIG. 1 is shown in FIG. 2 in conjunction with a number of alternative embodiments for pressing the web at two locations in the forming section of the paper machine. Although the foils 14, 24 and blades 20 are not employed in the embodiments of the present invention shown in FIG. 2, these devices may alternatively be used. The dashed box 100 designates the portion of the forming section where the web, sandwiched between the first and second forming wires 4, 6, are pressed in the nip N₁ formed by rolls 16 and 18. In the alternative embodiments of the present invention shown in boxes 100 a and 100 b, the roll nip may instead be an extended nip. In the embodiment shown in box 100 a, roll 116 a is an extended nip press roll which has a press shoe 117 a to press the web along a portion of the length of the web. In the embodiment shown in box 100 b, roll 118 b is an extended nip press roll which has a press shoe 117 b. In the embodiment shown in box 100 c, two press rolls 118 c form two press nips, N₁′, N₁″, with roll 116 c.

[0033] In FIG. 2 the dashed box 200 designates the portion of the forming section where the web, sandwiched between the second forming wire 6 and the fabric 26, are pressed in the nip N₂ formed by rolls 28 and 30. In the alternative embodiments of the present invention shown in boxes 200 a and 200 b, the roll nip N₂ may instead be an extended nip. In the embodiment shown in box 200 a, roll 230 a is an extended nip press roll which has a press shoe 229 a to press the web along a portion of the length of the web. In the embodiment shown in box 200 b, roll 228 b is an extended nip press roll which has a press shoe 229 b. In the embodiment shown in box 200 c, two press rolls 230 c form two press nips, N₂′ and N₂″ and with roll 228 c.

[0034] In accordance with the present invention, any one of the embodiments shown in boxes 100, 100 a, 100 b and 100 c of FIG. 2 may be used in conjunction with any of the embodiments shown in boxes 200, 200 a, 200 b, and 200 c.

[0035] In FIG. 3 is shown an alternative embodiment of the embodiment shown in FIG. 1. A suction box 34 is positioned along the common run of the two forming wires 4, 6 between roll 12 and roll 16. The suction box 34, which is positioned within the perimeter of the second forming wire 6 and proximate to the surface of the second forming wire 6, provides suction to the second forming wire 6 and the web, as indicated by the negative sign, “−”. An air pressure box 36 is positioned within the perimeter of the first forming wire 4 and extends along the common run of the two forming wires 4, 6. The air pressure box 36, which also extends partially around roll 16, has a plurality of air pressure zones which are preferably individually controlled to capable of providing varying degrees of air pressure, if desired. In particular, as shown in FIG. 3, all of the air pressure zones except for the final zone provide suction or negative pressure to the surface of the first forming wire 4, while the final zone provides a positive air pressure, as indicated by the positive sign, “+”. Roll 16 is also provided with a plurality of air pressure zones which are preferably individually controlled to capable of providing varying degrees of positive or negative air pressure, if desired. In FIG. 3, all of the air pressure zones of roll 16 provide positive air pressure to the second forming wire 6 except for the final zone which provides a negative pressure, or suction. A press roll 18 is optionally used in conjunction with roll 16 to press the web sandwiched between the two forming wires 4, 6. In operation, the air pressure zones aid in transporting the web along the desired path, and the negative pressure (or suction) zones additionally aid in removing water from the web and forming wires 4, 6. Alternatively, the optional press roll 18 may be replaced by an extended nip roll configuration as shown in boxes 100 a and 100 b of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, at nip N₁ the press shoe may be positioned in the lower roll as shown in box 200 a of FIG. 2, the nip may be a press nip as shown in box 200 of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0036] The embodiment of the present invention shown in FIG. 4 is substantially the same as that shown in FIG. 3 except that an initial felt 38 is introduced between roll 10 and wire 4 and partially wraps about roll 10. The felt 38 is guided by a number of guide rolls 8. A suction box 40 is positioned within the perimeter of felt 38 and proximate the surface of roll 10. Suction box 40 has a plurality of suction zones, designated by a negative sign, “−”. These suction zones are positioned proximate the point where the felt 38 begins and ends its partial wrap about roll 10. Roll 10 has a positive air pressure zone proximate the suction box 40. Felt 38 is used to aid in removing water from the web through the first forming wire 4. The suction zones of the suction box 40 and the positive air pressure zone of roll 10 aid in removing water that comes off of the felt 38. If, however, two-sided sheet properties are preferred, the felt 38 can be replaced by a non-water-receiving belt. In this case, no pressure or vacuum zones are needed. Alternatively, the optional press roll 18 may be replaced by an extended nip roll configuration as shown in boxes 100 a and 100 b of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, at nip N₂ the press shoe may be positioned in the lower roll as shown in box 200 a of FIG. 2, the nip may be a press nip as shown in box 200 of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0037] The embodiment of the present invention shown in FIG. 5 is similar to that shown in FIG. 3 with a few differences. The first forming wire 4, instead of the second forming wire 6, passes through the second nip N₂. In addition, the fabric 26 (i.e., a felt or belt) is applied to the web W immediately prior to the second nip N₂ so that the top surface of the fabric 26 is in contact with the web, rather than the bottom surface of the fabric 26. The second nip N₂ is formed by an extended nip press where the press shoe 229 b is in the bottom roll 230 b. Alternatively, the optional press roll 18 may be replaced by an extended nip roll configuration as shown in boxes 100 a and 100 b of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, at nip N₂ the press shoe may be positioned in the upper roll as shown in box 200 a of FIG. 2, the nip may be a press nip as shown in box 200 of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0038] In the embodiment of the present invention shown in FIG. 6, the first forming wire 4 passes only through the first nip, N₁, while the second forming wire 6 passes partially around roll 16 and through the second nip N₂. Roll 12 is completely eliminated, and the stock suspension is ejected substantially horizontally from the head box 2 between the converging surfaces of the first and second wires 4, 6 as they travel between rolls 10, 16 which bring the two forming wires 4, 6 together. A suction box 40 is positioned within the perimeter of forming wire 4 and proximate the surface of roll 10. Roll 10 has a positive air pressure zone proximate the suction box 40. The suction zones of the suction box 40 and the positive air pressure zone of roll 10 aid in removing water that comes off of the felt 38. In alternatives of this embodiment, the optional press roll 18 may be replaced by an extended nip roll configuration as shown in boxes 100 a and 100 b of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, in nip N₂ the press shoe may be positioned in the bottom roll as shown in box 200 bof FIG. 2, the nip may be a press nip as shown in box 200 of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0039] The embodiment of the present invention shown in FIG. 7 is substantially the same as that shown in FIG. 6 except that the first forming wire 4, rather than the second forming wire 6, passes through the second nip N₂. Consequently, the web is applied to the top surface of the felt rather than the bottom surface, as shown in FIG. 6. In addition, the head box 2 is positioned so that the paper pulp suspension is ejected in a substantially vertical direction. Finally, at the second nip N₂, the press shoe 229 b is located in the lower roll 228 b. In alternatives of this embodiment, the optional press roll 18 may be replaced by an extended nip as shown in boxes 100 a and 100 b of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, the press shoe may be positioned in the top roll as shown in box 200 a of FIG. 2, the nip may be a press nip as shown in box 200 of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0040] In the embodiment of the present invention shown in FIG. 8, only the second forming wire 6 passes through both nips, N₁ and N₂. The first forming wire 4 is used only in initially forming the web and to aid in transporting the web around roll 116 b. The stock suspension is ejected substantially horizontally from the head box 2 between the converging surfaces of the first and second wires 4, 6 as they travel between roll 116 b and guide roll 8 awhich bring the two forming wires 4, 6 together. Roll 116 b has a plurality of suction zones which aid in removing water from the web and in ensuring that the web remains in contact with roll 116 b as the first forming wire 4 is removed from the web before the web enters the first nip N₁. After the first forming wire has been removed from the web, a third forming wire 7 is applied to the web so that the web is sandwiched between the second forming wire 6 and the third forming wire 7 as they pass through the first nip N₁. The web sandwiched between the two forming wires 6, 7 passes through the second nip N₂. Then the second forming wire 6 is removed from the web, the fabric 26 is applied to the bottom surface of the web, and the third forming wire 7 is removed from the top surface of the web. The web, supported by the fabric 26 then travels to the drying section of the paper machine for further dewatering and drying. The position of a guide rolls 8 a located along the path of travel of the third forming wire 7 is preferably adjustable so that the tension in the wires 6, 7 can be adjusted. The extended nips N₁ and N₂ may alternatively be replaced with any of the roll configurations shown in boxes 100, 100 a or 100 c, and 200, 200 b or 200 c, respectively.

[0041] In the embodiment of the present invention shown in FIG. 9, both forming wires pass through both nips N₁ and N₂. The stock suspension is ejected substantially horizontally from the head box 2 between the converging surfaces of the first and second wires 4, 6 as they travel between roll 10 and roll 12 which bring the two forming wires 4, 6 together. In this embodiment, the web, sandwiched between wires 4 and 6 pass through one or more initial press nips N₀ before travelling through nip N₁ formed by rolls 116 a and 118 a. The initial press nip N₀ may be formed as a roll nip by two rolls 302 a, 304 a as shown in box 300 a, or as an extended nip as shown in boxes 300 b and 300 c, with the press shoe in either the upper roll 304 b and a solid roll 302 b in the lower position, as shown in box 300 b, or with the press shoe in the lower roll 302 a and a solid roll 304 c in the upper position, as shown in box 300 c. The particular number and type of initial press nips No is selected to obtain a desired web dryness and structure. Thus, there may be one, two or more initial press nips N₁ which may be press nips, extended nips, or a combination thereof. In alternatives of this embodiment, first nip N₁ may be replaced by an extended nip with the press shoe in the supper roll as shown in box 100 b of FIG. 2, or there may be a single roll nip as shown in box 100 of FIG. 2, or there may be two roll nips as shown in box 100 c of FIG. 2. Alternatively, in the second nip N₂, there can be an extended nip as shown in boxes 200 a and 200 b of FIG. 2, or there may be two roll nips as shown in box 200 c of FIG. 2.

[0042] In the embodiment of the present invention shown in FIG. 10, the stock suspension is ejected substantially horizontally from the head box 2 between the converging surfaces of the first and second wires 4, 6 as they travel between roll 10 and roll 12 which bring the two forming wires 4, 6 together. In this embodiment, a suction box 40 is positioned within the perimeter of wire 6 and proximate the surface of roll 12. Suction box 40 has a plurality of suction zones, designated by a negative sign, “−”. These suction zones are positioned proximate the point where the wire 6 begins and ends its partial wrap about roll 10. Roll 12 has a positive air pressure zone proximate the suction box 40. The suction and pressure zones are aid in removing water from roll 12 and from the web. A suction box 34 is positioned along the common run of wires 4, 6 from rolls 10, 12 to roll 16. A suction box 36 is positioned proximate the surface of roll 16 and has a plurality of suction zones to aid in removing water from the web and to ensure that the web travels along with wire 6 to nip N₂. All of the pressure zones in suction box 36 are suction or negative pressure zones. Roll 16 has a plurality of pressure zones. The fabric 26 is applied to the upper surface of the web, and the web, sandwiched between fabric 26 and wire 6 passes through nip N₂. As shown in boxes 100, 100 a, 100 b and 100 b of FIG. 2, the optional nip N₁ may have various configurations, and nip N₂ may also have various alternative configurations as shown in boxes 200, 200 b and 200 c of FIG. 2.

[0043] The embodiment shown in FIG. 11 is similar to that shown in FIG. 10 except that wire 4 travels through nip N₂ rather than wire 6. Consequently, fabric 26 is applied to the bottom surface of the web as it passes through nip N₂ rather than the top surface of the web. Also, the position of the press shoe in nip N₂ is reversed from that shown in FIG. 10. The last difference relates to the pressure zones in roll 16 and in suction box 36. In FIG. 11, all of the pressure zones in roll 16 are positive except for the final zone, and all of the pressure zones in suction box 36 are negative except for the final zone which is positive.

[0044] Although in the foregoing embodiments various specific locations for pressure and suction zones of various rolls and air pressure boxes are disclosed, the pressure zones may be located in additional rolls and/or additional pressure boxes may be added. Furthermore, the specific pressures (positive or negative) of the pressure zones and the number of such pressure zones may be changed to improve the transportation of the web and/or to increase water removal. Also, the angle at which the suspension is ejected from the head box may be varied as desired. In addition, so that the tension of the forming wires may be adjusted, the positions of one or more of the guide rolls 8 within a circuit of the forming wires may be made to be adjustable. It must be understood that in any of the embodiments according to the present invention one of the forming wires can be substituted by a belt that is substantially not-water-receiving. The surface of such a belt may optionally have a wire-type structure. This alternative is especially beneficial when two-sided paper properties, and hence one-sided dewatering, is preferred.

[0045] In any of the above described embodiments of the present invention, any one or more of the particular press rolls employed may be selected so that the pressure applied to the web may be adjusted at a number of points across the width of the web, that is in the direction perpendicular to the direction of travel of the web through the paper machine. By making the pressure being applied to the web so adjustable, inconsistencies in the thickness of the web across its width can be substantially removed.

[0046] Preferably, the linear load in the pressure nips are between about 50 kN/m and about 2000 kN/m, and the web speed is from about 400 m/min to about 2700 m/min. Preferably, the linear load in the first pressure nip N₁ is less than that in the second pressure nip N₂.

[0047]FIG. 12 is a bar chart showing experimental results employing a prepressing unit substantially as the nip N₂ in FIG. 3 using a shoe press nip. The web was pressed between the wire and a non-water-receiving transfer belt. Sheets of newspaper grade paper with a basis weight of 50 g/m² were introduced at a web speed of 16.7 m/sec. The paper was introduced into the first shoe press nip with an incoming dryness of 10%. The linear load of the shoe presses for each trial run is shown within the bar of the chart. For the first run, the reference run, there was no linear load, that is the shoe press did not press the web to any extent. The linear load in the shoe press for run No. 11 was 150 kN/m, for run No. 14 the linear load was 1300 kN/m, for run No. 25 the load was 1700 kN/m, and for run No. 16.1 the load was 450 kN/m. The height of the bar, designated “Formation”, reflects how well the web was formed and is used as a measure of how evenly distributed the fibers of the web are—the lower the formation value, the more evenly distributed the fibers in the web, and the better the paper quality is. Formation was determined by measuring the mass of the web per area (g/m²) using a conventional formation meter employing a radiation source and a radiation detector. For each run, the web was sampled at about 100 random positions. A mean value was obtained and the standard deviation from that mean value is shown in the bar graph. FIG. 12 demonstrates that formation was improved upon the use of the formation configuration employing shoe presses in accordance with the present invention.

[0048]FIG. 13 shows the effect of web dryness at the press inlet and the linear load in the shoe press on web formation as tested using a forming process in accordance with the present invention Several test runs were performed under different combinations of conditions, namely, the dryness of the web prior to being pressed in the shoe press (incoming dryness) and the pressure applied to the web in the shoe press (linear load). The quality of formation, reflected by the formation values, was assessed by the same method described above. As in FIG. 12, the formation value reflects how well the web was formed in each test case—the lower the formation value, the more evenly distributed the fibers in the web and the better the paper quality. The formation quality is, under the test conditions, improved by decreasing the incoming dryness or increasing the linear load, or both.

[0049]FIG. 14 shows the results of a study on orientation anisotropy behavior in thickness direction of the paper sheet formed in accordance with the present invention. The study was conducted using the same sample as in Case 2 of the formation analysis shown in FIG. 13, where the dryness at the press nip inlet (incoming dryness) was 8%. Orientation anisotropy is defined as the ratio of major axis to minor axis of the ellipse describing the fibers' pattern of orientation relative to the machine direction. This ellipse can be measures in a variety of way, such as, for example, optically, with ultra sound or with depolarized light. The anisotropy value reflects the tendency of the fibers to be oriented in the direction, i.e., in the thickness direction of the web. In general for paper making, the lower the anisotropy value, the better the paper quality. The data shown in FIG. 14 shows a decreasing tendency of orientation anisotropy as a function of increased linear load.

[0050] The results of these and other trial runs demonstrate that the evenness of a paper web and thus formation is improved when the web is pre-pressed in the formation section of a paper making machine. Those studies also reveal that the improvements are more pronounced on the side of the web that carries a higher hydraulic pressure during pressing than the opposing side.

[0051] Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 

What is claimed is:
 1. A method for improving the structure of a paper web in a forming section of a paper making machine comprising: ejecting a suspension of paper pulp from a head box between a pair of forming fabrics so that a paper web starts to be formed between the forming fabrics; and continuing web formation by pressing the web between two fabrics by passing the fabrics and the web sandwiched therebetween through at least one pressure nip formed by a pair of rollers such that water in the web is partially removed from the web.
 2. The method of claim 1, wherein at least one of the forming fabrics passes through the at least one pressure nip.
 3. The method of claim 1, wherein the paper pulp ejected from the head box between the forming fabrics has a dry content above about 2%.
 4. The method of claim 3, wherein the paper pulp ejected from the head box between the forming, fabrics has a dry content above about 3%.
 5. The method of claim 3, wherein the paper pulp ejected from the head box between the forming fabrics has a dry content of less than about 12%.
 6. The method of claim 5, wherein the paper pulp ejected from the head box between the forming fabrics has a dry content of less than about 10%.
 7. The method of claim 1, wherein at least one of the fabrics passing through the at least one pressure nip is a wire.
 8. The method of claim 7, wherein both of the fabrics passing through the at least one pressure nip are wires.
 9. The method of claim 1, wherein one of the fabrics passing through the at least one pressure nip is a water absorbing felt.
 10. The method of claim 1, wherein one of the fabrics passing through the at least one pressure nip is a non-water absorbing belt.
 11. The method of claim 1, wherein the at least one pressure nip is formed by a pair of press rolls.
 12. The method of claim 1, wherein the a t least one pressure nip comprises an extended nip press.
 13. An apparatus for improving the structure of a paper web in a forming section of a paper making machine comprising: a pair of endless first fabrics supported so that a surface of one of said first fabrics travels proximate a surface of the other of said first fabrics along a portion of the length of said first fabrics; a head box positioned to eject a suspension of paper pulp therefrom between said first fabrics so that the ejected paper pulp starts to form a web between said first fabrics; a pair of endless second fabrics supported so that a surface of one of said second fabrics travels proximate a surface of the other of said second fabrics along a portion of the length of said second fabrics, said pair of second fabrics positioned to receive the paper web from said pair of first fabrics; and at least one pressure nip positioned to press the web between said second fabrics by passing said pair of second fabrics and the web sandwiched therebetween through said at least one pressure nip, the web being pressed such that water in the web is partially removed from the web, said at least one pressure nip being formed by a pair of rollers.
 14. The apparatus of claim 13, wherein at least one of said second fabrics is at least one of said first fabrics.
 15. The apparatus of claim 13, wherein neither of said second fabrics is either of said first fabrics.
 16. The apparatus of claim 13, wherein at least one of said second fabrics is a wire.
 17. The apparatus of claim 16, wherein both of said second fabrics are wires.
 18. The apparatus of claim 13, wherein one of said second fabrics is a water absorbing felt.
 19. The apparatus of claim 13, wherein one of said second fabrics is a non-water absorbing belt.
 20. The apparatus of claim 13, wherein said at least one pressure nip is formed by a pair of press rolls.
 21. The apparatus of claim 13, wherein said at least one pressure nip comprises an extended nip press.
 22. The apparatus of claim 14, wherein one of said second fabrics is a water absorbing felt.
 23. The apparatus of claim 14, wherein one of said second fabrics is a non-water absorbing belt. 